2020-12-29T16:21:10Z
Acceptance in OA@INAF
Interferometric observations of warm deuterated methanol in the inner regions of
low-mass protostars
Title
Taquet, V.; Bianchi, E.; CODELLA, CLAUDIO; Persson, M. V.; Ceccarelli, C.; et al.
Authors
10.1051/0004-6361/201936044
DOI
http://hdl.handle.net/20.500.12386/29304
Handle
ASTRONOMY & ASTROPHYSICS
Journal
632
Number
Astronomy
&
Astrophysics
https://doi.org/10.1051/0004-6361/201936044© ESO 2019
Interferometric observations of warm deuterated methanol in the
inner regions of low-mass protostars
?
V. Taquet
1, E. Bianchi
2,1, C. Codella
1,2, M. V. Persson
3, C. Ceccarelli
2, S. Cabrit
4, J. K. Jørgensen
5, C. Kahane
2,
A. López-Sepulcre
2,6, and R. Neri
61INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
e-mail: taquet@arcetri.astro.it
2IPAG, Université Grenoble Alpes, CNRS, 38000 Grenoble, France
3Department of Space, Earth, and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala,
Sweden
4LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, UPMC Université Paris 06, 75014 Paris,
France
5Niels Bohr Institute, University of Copenhagen, Øster Voldgade 5–7, 1350 Copenhagen K., Denmark 6Institut de Radioastronomie Millimétrique, 38406 Saint-Martin d’Hères, France
Received 7 June 2019 / Accepted 19 September 2019
ABSTRACT
Methanol is a key species in astrochemistry because it is the most abundant organic molecule in the interstellar medium and is thought to be the mother molecule of many complex organic species. Estimating the deuteration of methanol around young protostars is of crucial importance because it highly depends on its formation mechanisms and the physical conditions during its moment of formation. We analyse several dozen transitions from deuterated methanol isotopologues coming from various existing observational datasets obtained with the IRAM-PdBI and ALMA sub-millimeter interferometers to estimate the methanol deuteration surrounding three low-mass protostars on Solar System scales. A population diagram analysis allows us to derive a [CH2DOH]/[CH3OH] abundance ratio of 3–6%
and a [CH3OD]/[CH3OH] ratio of 0.4–1.6% in the warm inner (≤100–200 AU) protostellar regions. These values are typically ten times
lower than those derived with previous single-dish observations towards these sources, but they are one to two orders of magnitude higher than the methanol deuteration measured in massive hot cores. Dust temperature maps obtained from Herschel and Planck observations show that massive hot cores are located in warmer molecular clouds than low-mass sources, with temperature differences of ∼10 K. The comparison of our measured values with the predictions of the gas-grain astrochemical model GRAINOBLE shows that such a temperature difference is sufficient to explain the different deuteration observed in low- to high-mass sources. This suggests that the physical conditions of the molecular cloud at the origin of the protostars mostly govern the present-day observed deuteration of methanol and therefore of more complex organic molecules. Finally, the methanol deuteration measured towards young solar-type protostars on Solar System scales seems to be higher by a factor of ∼5 than the upper limit in methanol deuteration estimated in comet Hale-Bopp. If this result is confirmed by subsequent observations of other comets, it would imply that an important reprocessing of the organic material likely occurred in the solar nebula during the formation of the Solar System.
Key words. astrochemistry – molecular processes – ISM: abundances – ISM: molecules – submillimeter: ISM – stars: formation
1. Introduction
Low-mass protostars are known to be chemically rich with the detection of several dozen molecules (van Dishoeck et al. 1995; Cazaux et al. 2003; Herbst & van Dishoeck 2009;Caux et al. 2011;Caselli & Ceccarelli 2012;Jørgensen et al. 2016). A pro-cess of deuterium enrichment is also observed for several neutral species, in particular for molecules thought to be mostly formed in cold interstellar ices and then released in the warm gas sur-rounding protostars (Ceccarelli et al. 2014). So far, methanol and formaldehyde are the two molecules showing the highest deu-terium fractionations, up to 50%, or 5 orders of magnitude higher than the cosmic D/H elemental abundance ratio of 1.6 × 10−5
(Linsky 2003;Ceccarelli et al. 1998;Parise et al. 2006). Other species that are thought to be also mostly formed in interstellar ices, such as water, show lower deuteration (Coutens et al. 2012;
?
The reduced datacubes are only available at the CDS via anony-mous ftp tocdsarc.u-strasbg.fr(130.79.128.5) or viahttp:// cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/632/A19
Taquet et al. 2013a;Persson et al. 2014). These differences have been invoked to reflect different moments or different mecha-nisms of formation (Cazaux et al. 2011;Taquet et al. 2013a,2014; Ceccarelli et al. 2014).
Gas-phase chemistry is known to be inefficient for the for-mation of gaseous methanol (Geppert et al. 2006;Garrod et al. 2006). Instead, methanol has been found to be efficiently formed at the surface of cold interstellar grains in dark cloud condi-tions through the hydrogenation of CO and H2CO (Watanabe & Kouchi 2002; Rimola et al. 2014). During the collapse of the protostellar envelope, solid methanol is released into the gas phase when the temperatures exceed ∼100 K. The deuteration of methanol is thought to remain constant after its evapora-tion in the warm-gas phase of the inner protostellar regions because gas-phase chemistry is likely too slow to significantly alter the ratios during the lifetime of the protostar (Charnley et al. 1997;Osamura et al. 2004). As a consequence, the deuter-ation observed around low-mass protostars would be a fossil of its formation in the precursor dense cloud where interstellar
ices formed.Taquet et al.(2012a,2013a) showed that like other species, methanol deuteration is highly sensitive to the tempera-ture and the density during ice formation. A deuteration higher than 1% is favoured by a methanol formation in cold (T = 10 K) and dense (nH≥ 105cm−3) molecular cloud conditions.
Most measurements of the methanol deuteration carried out so far have been performed using single-dish millimeter facilities with beams larger than 1000 (or ∼2000 AU at a typical distance
of 200 pc). These large beams encompass the different compo-nents of the protostellar system: the small hot core where all the icy content is thermally evaporated, but also the cold external envelope and molecular outflows driven by the central source(s). The advent of modern millimeter interferometers, such as the IRAM Plateau de Bure Interferometer (IRAM-PdBI), now called the NOrthern Extended Millimeter Array (NOEMA), and the Atacama Large Millimeter/submillimeter Array (ALMA) now allows astronomers to zoom towards the inner protostellar regions, in the so-called hot corinos, to measure the deuteration of the whole icy content released in the gas phase. New mea-surements towards low-mass protostars within different nearby molecular clouds suggest lower methanol deuterations than those originally measured a decade ago with single-dish telescopes. Bianchi et al.(2017) derived a [CH2DOH]/[CH3OH] abundance
ratio of 2% in HH212, a low-mass protostar located in the Orion molecular cloud, whilstJørgensen et al.(2018) estimated a [CH2DOH]/[CH3OH] ratio of 7% in Source B of IRAS
16293-2422, a low-mass protobinary system located in the Ophiuchus cloud. In addition to being lower than previous single-dish mea-surements of 20–60%, they also differ within each other by a factor of ∼3. This difference might be due to different properties of the precursor clouds. The Orion molecular cloud is the clos-est massive star-forming region and is known to be more active and slightly warmer than the more quiescent Ophiuchus cloud. However, this assumption remains to be confirmed for sources located in other clouds.
This article presents an analysis of different published observational datasets from the IRAM-PdBI towards NGC 1333-IRAS 2A and -IRAS 4A, two bright Class 0 low-mass protostars located in the nearby Perseus molecular cloud. We also re-analyse partially published data from the ALMA towards HH212 in order to measure the [CH2DOH]/[CH3OH] and
[CH3OD]/[CH3OH] abundance ratios in their hot corinos. In
this article, we aim to investigate whether the warm methanol deuteration observed in hot cores is regulated by the dust tem-perature of precursor dark clouds. To this aim, we compare our data with interferometric observations towards more massive protostellar systems and with the predictions of a modern astrochemical model.
2. Observations
2.1. Observational details
The two low-mass Class 0 protostars IRAS 2A and IRAS 4A located in the NGC 1333 cloud at 299 ± 14 pc (Zucker et al. 2018) were observed with the IRAM PdBI at 143, 165, and 225 GHz. Observations at 143 and 165 GHz were carried out on 2010 July 20, July 21, August 1, August 3, November 24, and 2011 March 10, in the C and D configurations of the array. Observations at 225 GHz were performed on 2011 November 27 and 28, on 2012 March 12, 15, 21, and 27, and on 2012 April, 2 in the B and C configurations of the array. A more detailed description of the observational setups is provided in Taquet et al. (2015) and Persson et al. (2014) for configurations B and C, respec-tively. The amplitude calibration uncertainty is estimated to be
∼20%. The WIDEX correlator has been used at three frequency settings, providing a bandwidth of 3.6 GHz each with a spec-tral resolution of 1.95 MHz (4, 3.5, and 2.6 km s−1at 145, 165,
and 225 GHz, respectively). The data calibration and imaging were performed using the CLIC and MAPPING packages of the GILDAS software1. Continuum images were produced by averaging line-free channels in the WIDEX correlator before the Fourier transformation of the data. The line spectral cubes were then obtained by subtracting the continuum visibilities from the whole (line+continuum) datacube, followed by natural weighted cleaning of individual channels. For IRAS 4A, the baseline has also been flattened by importing the data cubes into CLASS and subtracting a polynomial function of low order to each individ-ual spectrum. Positions are given with respect to the continuum peaks of IRAS 2A and IRAS 4A1 located at α(J2000) = 03h28m
55.s57, δ(J2000) = +31◦14037.0022 and α(J2000) = 03h29m10.s52, δ(J2000) = +31◦13031.0006, respectively.
The HH212-MM1 (hereafter HH212) protostar located in the Orion B cloud at about 400 pc (Kounkel et al. 2017) was observed in Band 7 with ALMA using 34 12 m antennas between June 15 and July 19, 2014, during the Cycle 1 phase and 44 12 m antennas between October 6 and November 26, 2016, during the Cycle 4 phase. The maximum baselines for the Cycle 1 and 4 observations are 650 m and 3 km, respectively. In Cycle 1, the spectral windows between 337.1–338.9 GHz and 348.4–350.7 GHz were observed using spectral channels of 977 kHz (or 0.87 km s−1), subsequently smoothed to 1.0 km s−1to increase sensitivity. Calibration was carried out following
standard procedures, using quasars J0607-0834, J0541-0541, J0423-013, and Ganymede. For the Cycle 4 data, a single spectral window between 334.1–336.0 GHz with a 488 kHz (or 0.42 km s−1) spectral resolution, successively smoothed to 1 km s−1, was used. Calibration was carried out using
quasars J0510+1800, J0552+0313, J0541–0211, and J0552–3627. Spectral line imaging was achieved using CASA2, whilst data analysis was performed using the GILDAS package. Positions are given with respect to the MM1 protostar continuum peak located at α(J2000) = 05h43m51.s41, δ(J2000) = –01◦02053.0017. In addition to13CH
3OH and CH2DOH images that have been
presented byBianchi et al.(2017), we also report the imaging of the CH3OD(62−–61+) transition at 335.1 GHz (see alsoCodella et al. 2019). The size of the synthesised beams and the rms noise are reported in Table1.
2.2. Spectroscopic parameters
We here focus on the CH2DOH, CH3OD, and CHD2OH
deuter-ated isotopologues. We used the CH2DOH JPL data entry from Pearson et al. (2012). The new band strengths published by Pearson et al. (2012) can differ by a factor of a 1.5–2.0 from the values fromParise et al.(2002) that are used in most previ-ous studies before 2012. With a vibrational correction factor of 1.15 at 160 K and 1.46 at 300 K based on the torsional data of Lauvergnat et al.(2009), the partition function is 1.3–2.0 higher than the partition function estimated by Parise et al. (2002). We re-analysed the CH2DOH transitions observed towards
the IRAS 16293-2422 low-mass protostellar system with the IRAM 30-m telescope by Parise et al. (2002) to investigate the effect of these new spectroscopic data on the estimation of the CH2DOH column densities. We derived a CH2DOH
col-umn density 2.2 lower than the one found by these authors. For CH3OD, we prepared a catalogue entry based on the band
1 http://www.iram.fr/IRAMFR/GILDAS 2 http://casa.nrao.edu
Table 1. Parameters of the observational data. NGC 1333-IRAS 2A (PdBI) Frequency 145 GHz 165 GHz 225 GHz Beam size (00) 2.1 × 1.7 2.3 × 1.7 1.2 × 1.0 Beam PA (◦) 25 110 22 rms(spectrum)(a) 2.6 3.5 2.9 NGC 1333-IRAS 4A (PdBI) Frequency 145 GHz 165 GHz 225 GHz Beam size (00) 2.2 × 1.7 2.4 × 1.8 1.1 × 0.8 Beam PA (◦) 25 114 14 rms(spectrum)(a) 3.3 4.0 4.8 HH212 (ALMA) 335 GHz 338 GHz 349 GHz Beam size (00) 0.15 × 0.12 0.52 × 0.34 0.41 × 0.33 Beam PA (◦) −88 −63 −63 rms(spectrum)(b) 1 5–6 5–6
Notes. (a)Units of mJy beam−1 channel−1 for a channel width of
1.95 MHz. (b)Units of mJy beam−1 channel−1 for a channel width of
0.87 km s−1(335 GHz) and 1.0 km s−1(at 338 and 349 GHz).
strengths published by Anderson et al. (1988) with frequen-cies updated byDuan et al.(2003). A self-consistent calculation of the CH3OD partition function has not be performed so far
because the CH3OD spectrum has only been partially studied in
the laboratory. Instead, we used two estimates for the CH3OD
partition function Z to derive the CH3OD column densities. The
first function follows the estimation by Parise (2004) within the rigid-rotor approximation, giving Z = 1.42T3/2 (see also Ratajczak et al. 2011), as used in most previous studies. The second follows the estimation byBelloche et al.(2016), whose partition function values are scaled with those of the CH18
3 OH
partition function (see also Jørgensen et al. 2018). At 150 K, the partition function estimated by Parise(2004) is four times lower than the one estimated byJørgensen et al.(2018), result-ing in large differences in the derived CH3OD column densities.
For CHD2OH, we used the spectroscopic data and the partition
function published byParise et al.(2002). The CHD2OH
parti-tion funcparti-tion is found to be similar to that of CH2DOH published
by the JPL database, with differences lower than 20% between 10 and 300 K.
We also targeted transitions from CH2DCN. The CH2DCN
spectroscopic data were taken from the CDMS catalogue, based onNguyen et al.(2013).
3. Results
3.1. Integrated maps
For IRAS 2A and IRAS 4A, the integrated emission maps of all methanol isotopologue transitions were obtained by integrating the flux over Vsys ±∆V, where Vsys is the source systemic
velocity and ∆V is the transition full width at half-maximum (FWHM) line width. In practice, we integrated the line emission over three channels because the spectral resolution of the WIDEX data is low. Figures 1 and 2 give an overview of the maps obtained for CH2DOH, CH3OD, and CHD2OH in the
three frequency settings. For all isotopologues, the emission is limited to the inner regions around the protostars within the
synthesised beams. Towards IRAS 4A, the molecular emission originates from the north-western source IRAS 4A2, although the continuum emission of the sourth-eastern source IRAS 4A1 is brighter. These maps are in good agreement with previous interferometric observations of these sources, which suggest that the emission of complex organic molecules originates from hot corinos whose sizes are smaller than 0.005 (or 150 au;Jørgensen et al. 2005;Persson et al. 2012;Maury et al. 2014;Taquet et al. 2015;López-Sepulcre et al. 2017;De Simone et al. 2017).
For HH212, we produced the integrated emission maps of 13CH3OH, CH2DOH, and CH3OD transitions that are not
blended with other transitions by integrating the flux over the entire line width after an analysis of their spectra towards the continuum peak. The Cycle 1 emission is spatially unre-solved with a a beam of 0.005 × 0.003, corresponding to a size of
225 × 135 au. However, the analysis of the higher angular reso-lution of Cycle 4 dataset provides a beam-deconvolved FWHM size of 0.0018 × 0.0012 (81 × 54 au) for the13CH3OH emission and
0.0020 × 0.0010 (90 × 45 au) for CH
2DOH. As shown in Fig.3, the
emission originates from the hot-corino region and is confined to a rotating structure that extends at ±45 au from the equato-rial plane and is elongated along and rotates around the jet axis. The actual interpretation of this emission is still debated and has been attributed to disc wind or a disc atmosphere from accretion shocks (Bianchi et al. 2017;Lee et al. 2017).
We measured the flux of all methanol isotopologue transi-tions integrated over an elliptical mask with similar size and position angle to the synthesised beam and with a centre at the molecular emission peak following the method described in Taquet et al.(2015). TablesA.1–A.6summarise the properties and the measured flux of the targeted transitions towards the two sources.
3.2. Population diagrams
We used the so-called population diagram (PD) method described in Goldsmith et al. (1999) and Taquet et al. (2015) to investigate the effect of optical depth on the column densi-ties of each level for the different methanol isotopologues. We performed a reduced chi-square (χ2red) minimization by running a grid of models covering a large parameter space in rotational temperature Trot between 50 and 350 K, and total column
den-sity in the source Ntotbetween 1015and 1020cm−2. We assumed
the source sizes θs derived in Taquet et al.(2015) andBianchi et al.(2017). A source size of 0.0036 and 0.0020 in IRAS 2A and IRAS 4A, respectively, was needed to reproduce the scatter of the population distribution of the low CH3OH upper energy
levels (seeTaquet et al. 2015), and a source size of 0.0019 was
derived by Bianchi et al.(2017) from the analysis of the spa-tially resolved13CH
3OH Cycle 4 emission map. At local thermal
equilibrium (LTE), the column density of every upper state Nup
can be derived for each set of Ntot, Trot, and source solid angle
Ωs. The best-fit model populations are plotted together with the
observed populations of the levels in Figs.4–6and are marked by crosses. Table2summarises the parameters of the best-fit mod-els and their associated uncertainties. The column densities of CH3OH and CH3CN have been derived inTaquet et al.(2015)
by analysing the emission from 28 CH3OH and 13 13CH3OH
transitions assuming a12C/13C elemental ratio of 70 and from
six CH3CN transitions detected in the 145 and 165 GHz
fre-quency settings, respectively.Taquet et al.(2015) used the same PD method by considering the rotational temperature Trot, the
total column density in the source Ntot, and the source size θSas
Continuum: 145.0 GHz
31°14'40" 37" 35" Dec. [J2000]CH
2DOH: 144.762 GHz CH
3OD: 143.742 GHz
Continuum: 165.0 GHz
31°14'40" 37" 35" Dec. [J2000]CH
2DOH: 166.063 GHz
CHD
2OH: 166.435 GHz
3h28m55.8s 55.6s 55.3s R.A. [J2000]Continuum: 225.0 GHz
3h28m55.8s 55.6s 55.3s 31°14'40" 37" 35" R.A. [J2000] Dec. [J2000]CH
2DOH: 225.878 GHz
3h28m55.8s 55.6s 55.3s R.A. [J2000]CH
3OD: 226.538 GHz
3h28m55.8s 55.6s 55.3s R.A. [J2000]Fig. 1. Integrated continuum maps, CH2DOH, CH3OD, and
CHD2OH emission observed
towards IRAS 2A for the three PdBI frequency settings (top: 145 GHz, middle: 165 GHz, and bottom: 225 GHz). Red contours show the 3σ and 6σ levels, whilst blue contours are in steps of 9σ. The synthesised beams are shown in the bottom left corner of each panel. The black cross depicts the position of the protostar.
Continuum: 145.0 GHz
31°13'33" 31" 28" Dec. [J2000]CH
2DOH: 144.762 GHz CH
3OD: 143.742 GHz
Continuum: 165.0 GHz
31°13'33" 31" 28" Dec. [J2000]CH
2DOH: 166.063 GHz
CHD
2OH: 166.435 GHz
3h29m10.8s 10.5s 10.3s R.A. [J2000]Continuum: 225.0 GHz
3h29m10.8s 10.5s 10.3s 31°13'33" 31" 28" R.A. [J2000] Dec. [J2000]CH
2DOH: 225.878 GHz
3h29m10.8s 10.5s 10.3s R.A. [J2000]CH
3OD: 226.538 GHz
3h29m10.8s 10.5s 10.3s R.A. [J2000]Fig. 2. Integrated continuum maps, CH2DOH, CH3OD, and CHD2OH
emission observed towards IRAS 4A for the three PdBI frequency settings (top: 145 GHz, middle: 165 GHz, and bottom: 225 GHz). Red contours show the 3σ and 6σ levels, whilst blue contours are in steps of 9σ. The synthesised beams are shown in the bottom left corner of each panel. The black crosses depict the position of the protostars.
Continuum: 335.0 GHz
5h43m51.5s 51.4s 51.4s -1°02'52.7" 53.2" 53.7" R.A. [J2000] Dec. [J2000]CH
2DOH: 334.683 GHz
5h43m51.5s 51.4s 51.4s R.A. [J2000]CH
3OD: 335.09 GHz
5h43m51.5s 51.4s 51.4s R.A. [J2000]Fig. 3. Integrated continuum maps, CH2DOH, and
CH3OD emission observed towards HH212 for the
ALMA frequency setting at 335 GHz. For the con-tinuum map, red contours show the 5 first 20σ levels and blue contours are in steps of 100σ. For the molecular maps, blue contours are in steps of 3σ. The synthesised beams are shown in the bottom left corner of each panel.
Table 2. Results from the PD analysis for the methanol and methyl cyanide deuterated isotopologues.
Molecule Nhc Trot Source size N(XD)/N(XH)(a) N(XD)/N(XH)(b)
(cm−2) (K) (00) (%) (Single-dish, %) IRAS 2A CH3OH (5.0+2.9−1.8)(+18) 140+20−20 0.36+0.04−0.04(c) – – 13CH 3OH (7.1+4.2−2.6)(+16) 140+20−20 0.36+0.04−0.04 (c) – – CH2DOH (2.9+1.2−0.6)(+17) 166−48+58 0.36(d) 5.8+1.3−1.5 62+71−33 CH3OD (P04(e)) (7.9+2.1−1.6)(+16) 166 ( f ) 0.36(d) 1.6+0.6 −0.8 ≤ 8 CH3OD (J18(e)) (3.6+0.9−0.4)(+17) 166( f ) 0.36(d) 7.1+1.2−1.6 ≤ 8 CHD2OH (2.2+0.6−0.5)(+17) 164+44−32 0.36 (d) 4.4+0.9 −1.3 25+29−14 CH3CN (2.0+1.2−0.4)(+16) 130+230−40 0.36(c) – – CH2DCN (7.2+1.5−0.9)(+14) 130(g) 0.36(g) 3.6+0.5−1.2 – IRAS 4A CH3OH (1.6+0.6−0.8)(+19) 140+30−30 0.20+0.08−0.04(c) – – 13CH 3OH (2.3+1.3−1.1)(+17) 140+30−30 0.20+0.08−0.04 (c) – – CH2DOH (5.9+2.2−1.8)(+17) 152−78+62 0.20(d) 3.7+1.2−0.9 65+30−21 CH3OD (P04(e)) (1.1+0.5−0.2)(+17) 152( f ) 0.20(d) 0.7+0.5−0.4 4.7+2.9−2.1 CH3OD (J18(e)) (5.0+1.3−1.0)(+17) 152( f ) 0.20(e) 3.1+1.0−0.8 4.7+2.9−2.1 CHD2OH (3.3+0.9−0.7)(+17) 138−38+34 0.20(d) 2.1+0.8−0.6 17+9−6 CH3CN (6.3+3.6−1.3)(+16) 200+110−40 0.20 (c) - -CH2DCN (1.7+0.7−0.7)(+15) 200(g) 0.20(g) 2.7+0.7−1.1 -HH212 13CH 3OH (3.2+1.4−1.4)(+16) 155+69−65 0.19(h) - -CH2DOH (6.4+1.3−1.3)(+16) 155(i) 0.19(h) 2.9+0.8−0.8 -CH3OD (P04(e)) (9.9+1.2−1.2)(+15) 155(i) 0.19(h) 0.4+0.3−0.3 -CH3OD (J18(e)) (4.4+0.5−0.5)(+16) 155(i) 0.19(h) 2.0+0.6−0.6
-Notes.(a)Column density ratio between a deuterated species and its main isotopologue.(b)FromParise et al.(2006).(c)FromTaquet et al.(2015). (d)The source size was assumed to be equal to that of CH
3OH.(e)P04 refers to the partition function computed byParise(2004) within the
rigid-rotor approximation, whilst J18 refers to the partition function used inJørgensen et al.(2018) obtained from a scaling of that of CH18
3 OH.
( f )The
rotational temperature was assumed to be equal to that of CH2DOH.(g)The rotational temperature was assumed to be equal to that of CH3CN. (h)FromBianchi et al.(2017).(i)The rotational temperature was assumed to be equal to that of13CH
3OH.
For IRAS 2A and IRAS 4A, we started the analysis with CH2DOH because this is the isotopologue with the highest
number of detected transitions and with the widest excitation range, with 25 detected transitions of upper level energies Eup
between 33 and 364 K. The population distributions can be reproduced with rotational temperatures of 166+58−48and 152+62−78K in IRAS 2A and IRAS 4A, respectively. These temperatures are slightly higher, but within the uncertainties, than the temperature of 140 K needed to reproduce the CH3OH and13CH3OH
emis-sions (Taquet et al. 2015). Transitions with upper level energies lower than 100 K are optically thick with opacities of 0.2–0.4 towards IRAS 2A and 0.5–1.0 towards IRAS 4A, depending on their properties. For CH3OD, we assumed the same
rota-tional temperature as for CH2DOH because of the lower range
of excitation (Eup from 33 to 104 K) and the high scatter and
uncertainties in the population distributions for both sources. The strong scatter of the CH3OD population is not entirely
reproduced, possibly because of non-LTE effets due to high
critical densities. CH3OD transitions are also optically thick with
opacities ranging from 0.3 to 1.3 towards the two sources. The CHD2OH population distribution shows much less scatter that
CH3OD. The comparison of the column densities derived in
this work with those found for CH3OH inTaquet et al.(2015)
allows us to derive the methanol deuterations in the two sources. We find a [CH2DOH]/[CH3OH] abundance ratio of 5.8 ± 0.8
and 3.2 ± 0.7% towards IRAS 2A and IRAS 4A, respectively. Using the CH3OD partition function estimated byParise(2004),
we find [CH2DOH]/[CH3OD] abundance ratios of 3.6 ± 1.6 and
5.3 ± 3.4 in IRAS 2A and IRAS 4A, respectively. However, as expected, we find lower [CH2DOH]/[CH3OD] abundance ratios
of 0.8 ± 0.3 and 1.2 ± 0.5 in IRAS 2A and IRAS 4A, respec-tively, when the partition function fromJørgensen et al.(2018) is used. As discussed in Sect.4.3, a [CH2DOH]/[CH3OH]
abun-dance ratio of ∼3 is consistent with the statistical value, whereas a ratio of ∼1 remains puzzling. In addition to methanol, we also detect three transitions from the deuterated methyl cyanide
0
50
100 150 200 250 300 350
E
up[K]
26
27
28
29
30
31
32
ln(
N
up/g
up)
IRAS2A
CH
2DOH, T
rot= 166 ± 53 K
CH
3OD, T
rot= 165 K
CHD
2OH, T
rot= 164 ± 38 K
Fig. 4.Rotational and population diagrams of deuterated methanol iso-topologues (CH2DOH, CH3OD, and CHD2OH) derived towards NGC
1333-IRAS 2A for source sizes derived from the PD analysis of the methanol population distribution (0.00
36; seeTaquet et al. 2015). Red, green, and blue symbols represent transitions at 145, 165, and 225 GHz, respectively.The CH3OD and CHD2OH levels have been artificially
shifted by −1.5 and −4 for clarity. Observational data are depicted by diamonds. Crosses show the best fit of the PD to the data. Blue and green symbols represent transitions at 2 and 1.3 mm, respectively. Error bars are derived assuming a calibration uncertainty of 20% in addi-tion to the statistical error. Black straight lines represent the best fit of the rotational diagram analysis to the data, assuming optically thin emission.
0
50
100 150 200 250 300 350
E
up[K]
27
28
29
30
31
32
33
ln(
N
up/g
up)
IRAS4A
CH
2DOH, T
rot= 153 ± 70 K
CH
3OD, T
rot= 152 K
CHD
2OH, T
rot= 138 ± 35 K
Fig. 5.Rotational and population diagrams of deuterated methanol iso-topologues (CH2DOH, CH3OD, and CHD2OH) derived towards NGC
1333-IRAS 2A for source sizes derived from the PD analysis of the methanol population distribution (0.00
20; seeTaquet et al. 2015). Red, green, and blue symbols represent transitions at 145, 165, and 225 GHz, respectively. The CH3OD and CHD2OH levels have been artificially
shifted by −1.5 and −4 for clarity. Observational data are depicted by diamonds. Crosses show the best fit of the PD to the data. Blue and green symbols represent transitions at 2 and 1.3 mm, respectively. Error bars are derived assuming a calibration uncertainty of 20% in addi-tion to the statistical error. Black straight lines represent the best fit of the rotational diagram analysis to the data, assuming optically thin emission.
0
100
200
300
400
500
E
up[K]
24
25
26
27
28
29
30
ln(
N
up/g
up)
HH212
13CH
3OH, T
rot= 155 ± 67 K
CH
2DOH, T
rot= 155 K
CH
3OD, T
rot= 155 K
Fig. 6.Rotational and population diagrams of methanol isotopologues (13CH
3OH, CH2DOH, and CH3OD) derived towards HH212 for the
source sizes derived from the ALMA Cycle 4 data (0.00
19; seeBianchi et al. 2017). The CH2DOH and CH3OD levels have been artificially
shifted by −3 for clarity. Red, green, and blue symbols represent tran-sitions at 335, 338, and 349 GHz, respectively. Observational data are depicted by diamonds. Crosses show the best fit of the PD to the data. Black straight lines represent the best fit of the rotational diagram analysis to the data, assuming optically thin emission.
isotopologue CH2DCN at 225 GHz. We obtain the CH2DCN
column densities by assuming that the CH2DCN rotational
tem-perature is equal to 200 K, the rotational temtem-perature of CH3CN
measured inTaquet et al.(2015). Comparing the CH2DCN
col-umn densities derived in this work with those of CH3CN from Taquet et al. (2015) allows us to obtain [CH2DCN]/[CH3CN]
ratios of 3.6 ± 0.8 and 2.7 ± 0.9% in IRAS 2A and IRAS 4A, respectively.
For HH212, we re-derived the column densities of13CH 3OH
and CH2DOH that have been estimated byBianchi et al.(2017).
Our method differs slightly from that of Bianchi et al. (2017) because the PD diagram is built upon the flux extracted from an elliptical mask instead of the continuum peak position in order to use consistent methods for the different sources. Assum-ing a source size of 0.0019 following Bianchi et al. (2017), we
started with 13CH3OH and then used the derived rotational
temperature to estimate the CH2DOH and CH3OD column
den-sities. We obtain a [CH2DOH]/[CH3OH] abundance ratio of
2.9 ± 0.8%. The [CH2DOH]/[CH3OD] abundance ratio is found
to be 7.2 ± 5.3 and 1.5 ± 0.6 with the CH3OD partition
func-tion estimated by Parise (2004) and Jørgensen et al. (2018), respectively.
4. Discussion
4.1. Comparison with previous observations
Deuterated methanol has previously been detected with the IRAM 30-m single-dish telescope towards IRAS 2A and IRAS 4A. Parise et al. (2006) derived [CH2DOH]/[CH3OH]
abundance ratios of 62 and 65% towards IRAS 2A and IRAS 4A, respectively, which are 10 and 17 times higher than the deuter-ation values found in this study. The [CH3OD]/[CH3OH] and
[CH2DOH]/[CH3OD] abundance ratios derived in this work
Sect.2.2). With the same partition function asParise(2004), the [CH3OD]/[CH3OH] ratio of 4.7% found byParise et al.(2006)
towards IRAS 4A is 6.7 times higher than our derived value, whilst the [CH2DOH]/[CH3OD] is found to decrease by a
fac-tor of 4 betweenParise et al.(2006) and this work. For doubly deuterated methanol, the [CHD2OH]/[CH3OH] ratios derived in
this work are also much lower than the abundances derived by Parise et al.(2006): by a factor of 5.6 and 8.1 for IRAS 2A and IRAS 4A, respectively.
The large differences found betweenParise et al.(2006) and the present work can be explained by several factors. First, the new CH2DOH spectroscopic data byPearson et al. (2012) used
in this work decrease by a factor of two the derived CH2DOH
column density following a re-analysis of the single-dish data towards IRAS 2A byParise et al.(2006) using the most recent spectroscopy data entry. This also naturally explains the increase of the [CHD2OH]/[CH2DOH] ratio in this work by a factor
of about two, from 41 to 75% in IRAS 2A and from 26 to 56% in IRAS 4A. Second, the methanol deuteration derived from single-dish observations byParise et al.(2006) has been estimated from the detection of various transitions from the main CH3OH isotopologue. However, the targeted transitions
have low upper level energies (Eup < 150 K) and are optically
thick in the inner protostellar regions surrounding these two low-mass protostars (see Taquet et al. 2015). Therefore, the rotational diagram analysis, assuming optically thin emission, carried out in Parise et al. (2006) could have likely led to an underestimation of the CH3OH column density. Finally, the
IRAM 30-m telescope has a large beam of 9–3000 depending on
the frequency. As suggested by the low rotation temperatures of 55 and 27 K found byParise et al.(2006) for CH2DOH towards
IRAS 2A and IRAS 4A, respectively, the methanol emission would mostly come from the large external envelope and/or from the molecular outflows driven by the targeted sources where methanol deuteration is higher. Methanol is mostly produced through surface chemistry through CO hydrogenation as gas phase chemistry is known to be inefficient (Watanabe & Kouchi 2002;Geppert et al. 2006). Non-thermal evaporation processes could release a fraction of solid methanol formed in ices into the gas phase. As shown byTaquet et al.(2014), the deuteration of methanol and other molecules mostly produced in ices like water is higher in external protostellar envelopes than in the inner hot cores because the cold gaseous deuteration reflects the deuteration at the highly deuterated ice surface.
Deuterated methanol has not previously been targeted with single-dish facilities towards HH212. Our PD analysis gives a [CH2DOH]/[CH3OH] abundance ratio of 2.9 ± 0.8%, which
agrees well with the value of 2.4 ± 0.4% found byBianchi et al. (2017), who used a slightly different method.
4.2. Deuteration from low- to high-mass protostars
Figure 7 and Table 3 compare the deuteration of CH2DOH,
CH3OD, and CH2DCN, including their statistical (i.e. taking
into account the statistical ratios for the functional groups with multiple H-atoms) correction, between four low-mass, one intermediate-mass, and three massive hot cores obtained only through interferometric observations of methanol isotopologues. We also show the water deuteration for comparison when avail-able. Because the main CH3OH isotopologue is known to be
optically thick in hot cores, we only chose observations whose deuteration values were derived from 13CH
3OH or CH183 OH
column densities assuming12C/13C and16O/18O elemental ratios
of 70 and 560, respectively (Wilson & Rood 1994). IRAS 16293– 2422, a low-mass protobinary system located in the Ophiuchus molecular cloud, has been observed with ALMA byJørgensen et al. (2018) in the context of the PILS survey that covered the whole ALMA band 7 between 329.15 and 362.90 GHz (Jørgensen et al. 2016). NGC 7129 FIRS 2, an intermediate-mass protostar, has been observed with the IRAM-PdBI at 220 GHz byFuente et al.(2014). The methanol deuterium fractiona-tion towards the Orion KL massive star-forming region has been estimated by Peng et al. (2012) through interferometric obser-vations using the IRAM-PdBI at 103 and 225 GHz. The Sgr B2(N2) massive hot core has been observed byBelloche et al. (2016) in the context of the EMoCA ALMA survey of the whole 3 mm band. The NGC 6334 massive protocluster has recently been observed byBøgelund et al.(2018) through ALMA obser-vations at 300 GHz, and we selected the deuteration derived using the CH18
3 OH column density. The error bars of the CH3OD
deuterations derived in this work include the different column density values derived with the two partition functions used for CH3OD.
The dust temperature in the dark clouds surrounding proto-stars is mostly governed by the external interstellar radiation field with local variations due to bright stars and/or dense cores. All the hot cores considered in Fig.7and Table3are likely relatively young, that is, have a lifetime of 105yr at most, the dust temper-atures of their precursor dark clouds are therefore likely similar to the current observed values. The deuterium fractionation is plotted against the current dust temperature of their surround-ing cloud as measured by the Planck observatory for NGC 7129 FIRS2 and by the PACS and SPIRE instruments on board the HerschelSpace Observatory for other sources. The Ophiuchus, Perseus, and Orion molecular clouds have been observed with the Gould Belt Survey key program (André et al. 2010), the Sgr B2 region has been observed byEtxaluze et al.(2013) as part of the Hi-GAL key program (Molinari et al. 2010), whilst the NGC 6334 massive complex has been observed with the HOBYS key program (Motte et al. 2010;Russeil et al. 2013;Tige et al. 2017). To estimate the dust temperature, we extracted 10 arcmin maps surrounding the selected sources. We then generated temperature histograms of all pixels showing visual extinctions AV higher
than 3 mag when the H2 column density maps are available
(for the Ophiuchus, Perseus, and NGC 6334 clouds) or showing 353 GHz opacities higher than ten times the rms noise (for the Orion and NGC 7129 clouds). To derive a temperature represen-tative of the entire region, we filtered out the hot regions that are locally heated by stars or cold regions of dense cores by fitting the temperature histogram around the main peak with a Gaussian function. FigureB.1gives an example for the NGC1333 region containing IRAS 2A and IRAS 4A.
It is found that the statistical deuterium fractionations of CH2DOH and CH3OD tend to decrease from a few percent
to less than ∼0.1% with the dust temperature between ∼13 and ∼24 K. With the same partition function as estimated by Belloche et al. (2016) and Jørgensen et al. (2018), the [CH2DOH]/[CH3OD] abundance ratio seems to show a large
dispersion because it typically varies from 1–4 in low-mass pro-tostars to 0.1–2 in high-mass hot cores (0.8 ± 0.3 in IRAS 2A, 1.2 ± 0.5 in IRAS 4A, 3.9 ± 1.1 in IRAS 16293, 1.5 ± 0.6 in HH212, 1.7 in Sgr B2, 0.73 ± 0.27 in Orion KL, and 0.13 ± 0.03 in NGC6334-IMM1).
It has been shown that warm hot-core gas-phase chem-istry is likely too slow to significantly alter the deuteration after the evaporation of ices in hot cores (Charnley et al. 1997; Osamura et al. 2004). Thus, the methanol deuterium
10.0 12.5 15.0 17.5 20.0 22.5 25.0 27.5 30.0
10
410
310
210
1D/
H
(C
H
2DO
H)
LM
IM
HM
10.0 12.5 15.0 17.5 20.0 22.5 25.0 27.5 30.0
10
410
310
210
1D/
H
(C
H
3OD
)
LM
HM
10.0 12.5 15.0 17.5 20.0 22.5 25.0 27.5 30.0
T
dust[K]
10
410
310
210
1D/H (HDO)
LM
HM
10.0 12.5 15.0 17.5 20.0 22.5 25.0 27.5 30.0
T
dust[K]
10
410
310
210
1D/
H
(C
H
2DC
N)
LM
HM
Fig. 7. Statistical (i.e. taking into account the statistical ratios for the functional groups with multiple H-atoms) CH2DOH (top left), CH3OD
(top right), HDO (bottom left), and CH2DCN (bottom right) deuterium fractionations observed towards the hot core of a sample of low-mass
(red), intermediate-mass (blue), and high-mass (green) protostars with sub-millimeter interferometers as function of the dust temperature of the surrounding cloud measured with the Herschel or Planck telescopes (see values in Table3).
Table 3. Surrounding cloud dust temperatures and statistical deuterium fractionations observed towards protostellar hot cores.
Source Tdust D/H (CH2DOH) D/H (CH3OD) [CH2DOH]/ D/H (CH2DCN) D/H (HDO) Ref.
(K) (%) (%) [CH3OD] (%) (%) IRAS 16293-B 16.6 ± 1.1 2.4 ± 0.5 1.8 ± 0.4 3.9 ± 1.1 1.2 ± 0.3 0.046 ± 0.013 [1, 2, 3] IRAS 2A 13.1 ± 0.7 1.9 ± 0.5 1.6 − 7.1 0.8 − 3.6 1.2 ± 0.4 0.085 ± 0.040 [4, 5] IRAS 4A 13.1 ± 0.7 1.2 ± 0.4 0.7 − 3.1 1.2 − 5.3 0.90 ± 0.37 0.17 ± 0.08 [4, 6] HH212 15.3 ± 0.7 1.0 ± 0.3 0.4 − 2.0 1.5 − 7.2 – – [7, 4] NGC 7129 FIRS1 19.3 ± 0.2 0.21 ± 0.04 – – – – [8] Sgr B2 24 ± 1 0.04 0.07 1.7 0.13 – [9] Orion KL 24.3 ± 0.4 0.037 ± 0.007 0.15 ± 0.05 0.73 ± 0.27 – 0.13 ± 0.06 [10, 11] NGC 6334 IMM1I 23.9 ± 0.8 0.020 ± 0.007 0.48 ± 0.12 0.13 ± 0.03 – – [12]
Notes. For the CH3OD column densities in IRAS 2A, IRAS 4A, and HH212, two estimates are given using the partition functions estimated by
Parise(2004) andJørgensen et al.(2018). The highest CH3OD column densities (and the lowest [CH2DOH]/[CH3OD] ratios) are those with the
partition function estimated byJørgensen et al.(2018).
References. [1]Jørgensen et al.(2018); [2]Calcutt et al.(2018); [3]Persson et al.(2013); [4] this work; [5]Coutens et al.(2014); [6] average between HDO/H2O ratios of 5 × 10−3(Taquet et al. 2013a) and 5.4 × 10−4(Persson et al. 2014, M. Persson, priv. comm.); [7]Bianchi et al.(2017);
[8]Fuente et al.(2014); [9]Belloche et al.(2016); [10]Peng et al.(2012); [11]Neill et al.(2013); [12]Bøgelund et al.(2018).
fractionation observed in the warm gas-phase of low-mass and massive hot cores would rather reflect the deuteration of methanol formed in interstellar ices within their progenitor dense clouds.
4.3. Comparison with theoretical predictions
We compared the observed methanol deuterations with the pre-dictions of the GRAINOBLE astrochemical code by Taquet et al. (2012a, 2013a, 2014), which follows the formation and
the deuteration of ices in cold dense cores. The code has been extensively described in previous studies and has recently been used to interpret the deuteration towards NGC 6334 measured byBøgelund et al.(2018). We investigated the effect of the tem-perature, total density, and time on the methanol deuteration using the same code and same chemical network as in Taquet et al. (2014). However, unlike in Taquet et al. (2014), we ran here a series of pseudo-time-dependent simulations in which the chemistry evolves over time for constant physical properties such as temperature or density. FigureC.1 shows the deuteration of methanol and water in ices as a function of temperature between 10 and 30 K for three different dense cloud densities, nH= 104,
105, and 106 cm−3 at three different times, 0.1 × tFF, tFF, and
10 × tFF, where tFFis the free-fall time. tFFis equal to 4.4 × 105,
1.4 × 105, and 4.4 × 104yr at densities of 104, 105, and 106cm−3,
respectively.
The methanol deuteration strongly depends on the density, the considered temperature, and time. Methanol and its deuter-ated isotopologues are formed through addition reactions of atomic H and D on CO and H2CO on ices possibly
supple-mented by abstraction and substitution reactions (Nagaoka et al. 2005; Hidaka et al. 2007, 2009). The methanol deuteration is therefore governed by the atomic [D]/[H] abundance ratio in the gas phase during ice formation. Atomic D is mostly formed through electronic recombination of H+3 isotopologues, which in turn are formed through exothermic reactions between H+3 and HD. The efficiency of the backward reaction increases with increasing temperature and with the ortho/para ratio of H2. In
addition, reactions between H+3or with its isotopologues and HD are in competition with reactions involving CO. The production of atomic deuterium is therefore enhanced at low temperatures, close to 10 K, when the abundances of CO and ortho state of H2
are low (see Fig. 1 ofTaquet et al. 2012b, for a scheme detailing the gas-phase deuteration network). As a consequence, the strong decrease of up to two orders of magnitudes of the methanol deuteration predicted at t ≥ tFF from 10 to 30 K is essentially
due to the decreased efficiency of deuterium chemistry in the gas phase of cold dense cores.
The observed [CH3OD]/[CH3OH] ratios observed both
towards low-mass and massive protostars are relatively well reproduced by the model predictions at t ≥ tFF, despite the
large dispersion of values among massive protostars. The [CH2DOH]/[CH3OH] abundance ratios of low-mass protostars
can also be explained by the models at t ≥ tFF, but the
massive protostar values can only be explained by a shorter time of 0.1 × tFF or a low density of 104 cm−3. The
pre-dicted [CH2DOH]/[CH3OD] ratio remains close to the
statis-tical value of 3 in most cases and can increase up to ∼6 at 106 cm−3, T ∼ 10–15 K, and t = 10 × t
FF. Our model only
focuses on cold surface chemistry and is not able to explain [CH2DOH]/[CH3OD] ratios lower than 1 observed in massive
hot cores. CH3OD would then need to be produced through other
processes such as warm gas-phase chemistry following methanol evaporation within massive hot cores. Osamura et al. (2004) showed that the reaction H2DO+ + CH3OH → CH3OHD+ +
H2O, followed by dissociative recombination, could increase the
[CH3OD]/[CH3OH] ratio by a factor of ∼5 at 100 K. However,
this increase is obtained at times longer than 105yr and
assum-ing [HDO]/[H2O] abundance ratios of ∼0.1 that are about 100
times higher than the observed values. It remains to be tested whether this process could play a major role at higher temper-atures and for realistic water deuteration values. On the other hand, hydrogen-deuterium exchanges in warm ices have been proven to be efficient, but only on the hydroxyl functional group
of methanol. CH3OD would thus give its deuterium to water,
inducing a decrease in CH3OD abundance in ices before its
evaporation (Ratajczak et al. 2009;Faure et al. 2015).
Unlike methanol, water does not show any significant decrease of its observed deuteration with the dust temperature because it remains around 0.1% between 13 and 24 K. The low water deuteration has been interpreted as due to an early for-mation of solid water in the translucent phase at low visual extinctions, when the dark cloud is still lukewarm, with temper-atures of 15–20 K, and when the abundances of CO and ortho H2 are still high (Taquet et al. 2013a,2014). Methyl cyanide is
thought to be mostly produced either on warm ices from recom-bination between the CN and CH3 radicals or in the gas phase
through the radiative association between HCN and CH+3, after the prestellar stage. Comparing the methyl cyanide deuteration with the model predictions with constant physical conditions of cold cores is therefore not relevant.
4.4. Deuteration from low-mass protostars to comets
Solar System comets mostly contain pristine material that has poorly evolved since the end of the Solar System formation 4.5 billions years ago. Their chemical composition should reflect the chemical composition of the solar nebula (see Mumma & Charnley 2011, for instance). Comparing the deuteration of water and organics measured around young solar-type protostars on Solar System scales and in Solar System comets should allow us to follow the chemical evolution throughout the star formation process.
Figure 8 compares the (statistical) deuterium fractionation of methanol, methyl cyanide, and water observed in low-mass protostars with the deuteration of HCN in protoplanetary discs and of CH3OH, HCN, and water in comets. Deuteration of water
has been estimated in four nearby protostars on Solar System scales byTaquet et al.(2013a),Persson et al.(2014) andCoutens et al. (2014) through interferometric observations of HDO and H18
2 O transitions and about a dozen comets (see the list of
ref-erences in the caption of Fig.7). Water deuteration in protostars is only higher by about a factor of two than the deuteration mea-sured in comets. Unlike water, cometary deuterated methanol has not been detected so far. The most stringent upper limit in the methanol deuteration has been given byCrovisier et al.(2004) towards comet Hale-Bopp, with a [CH2DOH]/[CH3OH]
abun-dance ratio lower than 0.8%. The [CH2DOH]/[CH3OH] ratio of
3–6% estimated with interferometers towards young protostars is higher by a factor of 4–8 than the estimate made in comet Hale-Bopp. A low methanol deuteration in comets remains to be confirmed with a clear detection of deuterated methanol, either through the analysis of the high-resolution ROSINA mass spec-trometer data at mass 33 on board the Rosetta space probe (see for instanceLe Roy et al. 2015), or through future sensitive sub-millimeter or sub-millimeter ground-based surveys of nearby and bright comets using interferometers like ALMA or NOEMA.
Assuming that the observed deuteration towards nearby low-mass protostars is representative for the deuteration of the young Sun, this comparison suggests that an important reprocessing of the organic material occurred in the solar nebula whilst little water reprocessing in the solar nebula is required. Three reasons can be suggested. First, methanol observed in the hot core of protostars on ∼100 AU scales is reprocessed before entering in the disc mid-plane through high-temperature chemistry involv-ing reactions with H or OH because they have moderate energy barriers of 2200–3000 K (Li & and Williams 1996). However, the temperature of the hot corino is likely too low to trigger
1P/Halley
C/1996 B2 (Hyakutake)
C/1995 O1 (Hale-Bopp)
8P/Tuttle
C/2007 N3 (Lulin)
C/2002 T7 (LINEAR)
153P/Ikeya-Zhang
C/2009 P1 (Garradd)
C/2012 F6 (Lemmon)
C/2014 Q2 (Lovejoy)
67P/C-G
103P/Hartley 2
45P/HMP
AS 209
IM Lup
V4046 Sgr
LkCa 15
MWC 480
HD 163296
IRAS16293
IRAS2A
IRAS4A
IRAS4B
HH212
10
510
410
310
210
1(D
/H
)
xD/H (ISM)
H2O
H2O
HCN
HCN
CH
3CN
CH
3OH
CH
3OH
Oort-Cloud Comets
Jupiter-Family
Comets
Disks
Protostars
Fig. 8.Statistical D/H ratios in Solar System comets, protoplanetary discs, and low-mass Class 0 protostars for methanol (red), water (blue), and HCN or CH3CN (green). References: 1P/Halley:Eberhardt et al. (1995); C/1996 B2 (Hyakutake):Bockelée-Morvan et al.(1998); C/1995 O1
(Hale-Bopp):Meier et al.(1998);Crovisier et al.(2004); 8P/Tuttle:Villanueva et al.(2009); C/2007 N3 (Lulin):Gibb et al.(2012); C/2002 T7 (LINEAR):Hutsemékers et al.(2008); 153P/Ikeya-Zhang:Biver et al.(2006); C/2009 P1 (Garradd):Bockelée-Morvan et al.(2012); C/2012 F6 (Lemmon):Biver et al.(2016); C/2014 Q2 (Lovejoy):Biver et al.(2016); 67P/C-G:Altwegg et al.(2015); 103P/Hartley 2:Hartogh et al.(2011); 45P/HMP:Lis et al.(2013); All discs:Huang et al.(2017); IRAS 16293:Persson et al.(2014);Jørgensen et al.(2018);Calcutt et al.(2018); IRAS 2A:Coutens et al.(2014), this work; IRAS 4A: average values derived byTaquet et al.(2013b) andPersson et al.(2014, M. Persson priv. comm.), this work; IRAS4B:Persson et al.(2014).
such reactions and other processes that increase the gas tem-perature to ∼1000 K, such as accretion shocks that form at the interface of the envelope and the disc in formation (Aota et al. 2015), would be needed. Second, physical processes such as tur-bulence in the disc could transport the material from the disc mid-plane to the atmosphere where efficient photolytic processes can photo-dissociate organic molecules.Albertsson et al.(2014) and Furuya et al.(2014) demonstrated that radial and vertical turbulence can decrease the water deuteration by typically one order of magnitude in the cometary zone. However, it is yet to be demonstrated that this process could also affect the deuter-ation of more complex organic species with slightly different chemistries. A third possibility is that the cloud at the origin of the Solar System was initially slightly warmer than the tempera-tures of nearby dark clouds in which the observed protostars are located. This has recently been suggested byTaquet et al.(2016), who concluded that a dark cloud temperature of ∼20 K, together with a dark cloud density higher than 105 cm−3, was needed to explain the high abundance of O2 and its strong correlation
with water in comet 67P/CG. This modelling work confirmed the studies of meteoritic data, which suggest that the Solar System was born in a dense cluster of stars (seeAdams 2010). According to Fig.7, an increase in dark cloud temperature from 10 to 20 K would decrease the methanol deuteration from 2–6 to 0.2–0.6%, depending on the density, which seems to be in good agreement with the value found in comet 67P/CG.
5. Conclusions
We analysed several existing observational datasets obtained with the PdBI and ALMA sub-millimeter interferometers to estimate the methanol deuterations in the hot cores surround-ing three protostars on Solar System scales. For this purpose, we analysed several dozen deuterated methanol transitions with a PD analysis in order to measure the [CH2DOH]/[CH3OH]
and [CH3OD]/[CH3OH] abundance ratios. The obtained
[CH2DOH]/[CH3OH] ratios of 3–6% and [CH3OD]/[CH3OH]
ratios of 0.4–1.6% are typically one order of magnitude lower than previous estimates derived from single-dish observations towards the same sources, and are in good agreement with the recent ALMA measurements byJørgensen et al.(2018) towards the low-mass protostar IRAS 16293-B. We then compared our methanol deuteration estimates with previous measurements of intermediate- and high-mass hot cores with similar observa-tional properties and analysis methods. Unlike water, which does not show strong variation of its deuteration between low-mass and high-mass protostars, we find that the methanol deuteration around massive hot cores is much lower by one to two orders of magnitude. This strong difference could be attributed to a different physical or chemical history of the sources. Methanol observed around protostars is mostly formed in interstellar ices during the previous molecular cloud phase. Dust temperature maps derived with the Herschel or Planck space observatories
suggest that the observed massive protostars were born in warm molecular clouds of T ∼ 24 K, whilst low-mass protostars are located in more quiescent and cold regions with temperatures lower than 15 K. Comparing the observed deuteration values with the predictions of the GRAINOBLE astrochemical model on ice formation and deuteration, an increase of 10 K in the dust temperature is enough to explain a decrease of the observed methanol deuteration, depending on the density and the chem-ical time. Finally, the methanol deuterations measured towards young solar-type protostars at high angular resolution on Solar System scales seem to be higher by a factor of ∼5 than the upper limit in methanol deuteration estimated in comet Hale-Bopp by Crovisier et al.(2004). If this result is confirmed by subsequent observations of other comets, it would imply that an important reprocessing of the organic material likely occurred in the solar nebula during the formation of the Solar System.
Acknowledgements.This work is based on observations carried out with the ALMA Interferometer under project numbers ADS/JAO.ALMA#2012.1.00997.S and ADS/JAO.ALMA#2016.1.01475.S data (PI: C. Codella) and with the IRAM PdBI/NOEMA Interferometer under project numbers V05B and V010 (PI: M.V. Persson) and U003 (PI: V. Taquet). ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). V.T. acknowledges the financial support from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement n. 664931. E.B., Ce.Ce., C.K., A.L. and Cl.Co acknowledge the fund-ing from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme, for the Project “The Dawn of Organic Chemistry” (DOC), grant agreement No 741002. Cl.Co acknowl-edges the funding from PRIN-INAF 2016 “The Cradle of Life – GENESIS-SKA (General Conditions in Early Planetary Systems for the rise of life with SKA)”. Ce.Ce. and Cl.Co acknowledge the financial support from the European MARIE SKŁODOWSKA-CURIE ACTIONS under the European Union’s Horizon 2020 research and innovation programme, for the Project “Astro-Chemistry Origins” (ACO), Grant No 811312.
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Appendix A: Line parameters of the targeted transitions
Table A.1. Line parameters of CH2DOH lines observed towards IRAS 2A and IRAS 4A.
Number Frequency Transition Eup Aul Flux (IRAS 2A) Flux (IRAS 4A)
(GHz) (K) (s−1) (Jy km s−1) (Jy km s−1) 1 166.063164 22,0e1–11,0o1 33.0 2.40(−5) 0.285 ± 0.081 0.199 ± 0.076 2 166.787448 22,1e1–11,1o1 33.0 2.43(−5) 0.292 ± 0.071 0.184 ± 0.051 3 225.878232 31,3o1–20,2o1 35.6 3.23(−5) 0.607 ± 0.123 0.204 ± 0.056 4 226.818248 51,4e0–41,3e0 36.7 3.58(−5) 0.931 ± 0.187 0.349 ± 0.074 5 144.762399 41,3e1–30,3o1 38.2 8.80(−6) 0.132 ± 0.028 0.153 ± 0.043 6 164.108467 50,5e1–41,4e1 45.6 2.67(−6) 0.086 ± 0.031 0.090 ± 0.031 7 223.898819 32,2o1–42,2e1 48.3 1.56(−6) 0.450 ± 0.095 0.219 ± 0.068 8 225.667709 51,4e1–41,3e1 49.0 4.44(−5) 0.272 ± 0.062 0.351 ± 0.075 9 224.928016 51,4e2–41,3e2 55.3 4.42(−5) 1.390 ± 0.280 0.428 ± 0.092 10 144.134719 53,2e0–62,5e0 68.1 2.64(−6) 0.069 ± 0.026 0.098 ± 0.035 11 223.691457 53,3e0–43,2e0 68.1 2.17(−5) 0.601 ± 0.122 0.203 ± 0.043 12 223.697110 53,2e0–43,1e0 68.1 2.17(−5) 0.608 ± 0.124 0.217 ± 0.051 13 225.848108 70,7o1–61,6o1 78.3 2.17(−5) 0.604 ± 0.123 0.540 ± 0.114 14 165.861895 80,8e1–81, 7e0 90.4 8.99(−6) 0.217 ± 0.057 0.056 ± 0.020 15 164.577869 81,7e1–72,6e1 94.4 1.52(−6) 0.058 ± 0.023 0.071 ± 0.029 16 223.616196 54,2e0–44,1e0 95.1 1.26(−5) 0.522 ± 0.107 0.264 ± 0.065 223.616210 54,1e0–44,0e0 95.1 1.26(−5) 17 164.374775 104,7e0–103,7o1 181.0 5.91(−6) 0.073 ± 0.029 0.062 ± 0.031 18 164.388327 104,6e0–103,8o1 181.0 5.91(−6) 0.087 ± 0.032 0.106 ± 0.043 19 143.566661 122,10e0–113,9e0 184.5 3.76(−6) 0.082 ± 0.027 0.041 ± 0.016 20 224.273919 105,6e0–114,7e0 215.4 1.16(−5) 0.126 ± 0.035 0.082 ± 0.025 21 224.285457 105,5e0–114,8e0 215.4 1.16(−5) 0.178 ± 0.041 0.152 ± 0.038 22 166.950015 124,9e0–123,9o1 230.4 6.80(−6) 0.079 ± 0.031 0.065 ± 0.018 23 166.995876 124,8e0–123,10o1 230.4 6.80(−6) 0.072 ± 0.027 0.064 ± 0.021 24 142.838827 161,15o1–160,16o1 316.2 9.85(−6) 0.138 ± 0.031 0.099 ± 0.026 25 225.551640 172,15o1–171,16o1 363.6 3.28(−5) 0.273 ± 0.059 0.149 ± 0.042
Table A.2. Line parameters of CH3OD lines observed towards IRAS 2A and IRAS 4A.
Number Frequency Transition Eup Aul Flux (IRAS 2A) Flux (IRAS 4A)
(GHz) (K) (s−1) (Jy km s−1) (Jy km s−1) 1 226.53867 50+–40+A 32.7 4.31(−5) 0.869 ± 0.175 0.290 ± 0.064 2 226.350191 50–40E 36.4 4.66(−5) 0.744 ± 0.151 0.380 ± 0.084 3 226.185930 5−1–4−1E 37.3 4.16(−5) 0.433 ± 0.088 0.206 ± 0.051 4 143.741650 51−–50+A 39.6 3.52(−5) 0.237 ± 0.054 0.121 ± 0.029 5 226.92258 5−2–4−2E 50.2 3.71(−5) 0.355 ± 0.074 0.143 ± 0.035 6 226.892864 52–42E 54.4 3.71(−5) 0.267 ± 0.055 0.113 ± 0.025 7 226.94283 52+–42+A 54.4 3.70(−5) 0.436 ± 0.091 0.253 ± 0.067 8 226.825536 53–43E 70.5 3.71(−5) 0.254 ± 0.053 0.119 ± 0.026 9 226.70660 52−–42−A 100 2.84(−5) 0.300 ± 0.062 0.149 ± 0.040 10 226.73886 5−4–4−4E 104.3 1.60(−5) 0.073 ± 0.022 0.062 ± 0.031
Table A.3. Line parameters of CHD2OH lines observed towards IRAS 2A and IRAS 4A.
Number Frequency Transition Eup Aul Flux (IRAS 2A) Flux (IRAS 4A)
(GHz) (K) (s−1) (Jy km s−1) (Jy km s−1) 1 166.435 40–30e0 20.0 1.67(−5) 0.245 ± 0.066 0.151 ± 0.028 2 166.327 40–30o1 28.8 1.67(−5) 0.248 ± 0.107 0.129 ± 0.030 3 166.234 40–30e1 38.4 1.66(−5) 0.272 ± 0.064 0.139 ± 0.033 4 166.271 42−–32−e1 51.3 1.25(−5) 0.184 ± 0.066 0.106 ± 0.026 5 166.304 42+–32+e1 51.3 1.25(−5) 0.147 ± 0.045 0.074 ± 0.021 6 166.297 43+–33−e1 67.0 7.14(−6) 0.176 ± 0.055 0.048 ± 0.020 166.298 43−–33−e1 67.0 7.14(−6)
Table A.4. Line parameters of CH2DCN lines observed towards IRAS 2A and IRAS 4A.
Number Frequency Transition Eup Aul Flux (IRAS 2A) Flux (IRAS 4A)
(GHz) (K) (s−1) (Jy km s−1) (Jy km s−1) 1 224.754530 131,13–121,12 80.9 9.72(−4) 0.159 ± 0.032 0.081 ± 0.030 2 225.723769 133,11–133,10 124.4 9.38(−4) 0.130 ± 0.030 0.089 ± 0.018 3 225.724053 133,10–133,9 124.4 9.38(−4) 0.130 ± 0.030 0.089 ± 0.018 4 225.726540 133,12–133,11 97.4 9.68(−4) 0.162 ± 0.037 0.069 ± 0.018 5 225.781540 132,11–132,10 97.4 9.98(−4) 0.162 ± 0.037 0.069 ± 0.020
Table A.5. Line parameters of CH2DOH lines observed towards HH212.
Number Frequency Transition Eup Aul Flux (HH212)
(GHz) (K) (s−1) (Jy km s−1) 1 348.16076 41,3e1–40,4e0 38.1 2.03(−4) 0.152 ± 0.031 2 338.95711 61,6e0–50,5e0 48.4 1.59(−4) 0.157 ± 0.031 3 337.34866 90,9e0–81,8o0 96.3 1.46(−4) 0.159 ± 0.032 4 350.09024 54,2e1–53,2o1 104.2 7.58(−5) 0.065 ± 0.065 5 350.09038 54,1e1–53,3o1 104.2 7.58(−5) 0.065 ± 0.065 6 350.02735 64,3e1–63,3o1 117.1 9.06(−5) 0.078 ± 0.016 7 350.02777 64,2e1–63,4o1 117.1 9.06(−5) 0.078 ± 0.016 8 349.95168 74,4e1–73,4o1 132.1 1.00(−4) 0.201 ± 0.040 9 338.86898 131,12e0–120,12o1 201.8 2.81(−5) 0.080 ± 0.016 10 347.76728 184,15e1–183,15o1 438.2 1.31(−4) 0.046 ± 0.009 11 347.95281 184,14e1–183,16o1 438.2 1.31(−4) 0.057 ± 0.011
Table A.6. Line parameters of the CH3OD line observed towards HH212.
Number Frequency Transition Eup Aul Flux (HH212)
(GHz) (K) (s−1) (Jy km s−1)
1 335.089661 62−–61+ 67.4 1.65(−4) 0.025 ± 0.003
Appendix B: Column density and dust temperature maps
3h29m20s 00s 28m40s 20s 31°20'00" 15'00" 10'00" 05'00" R.A. [J2000] De c. [J2 00 0] 21.0 21.5 22.0 22.5 23.0 log (H2 co lum n d en si " [ cm −2]) 3h29m20s 00s 28m40s 20s 31°20'00" 15'00" 10'00" 05'00" R.A. [J2000] De c. [J2 00 0] 8 10 12 14 16 18 20 22 24 Te mp era tur e [ K] 8 10 12 14 16 18 20 22 24 Temperature [K] 0 500 1000 1500 2000 2500 3000 3500 4000 Nu mb er of pix els
Fig. B.1.H2column density (left), dust temperature (centre) maps, and dust temperature histogram of the map pixels with AV > 3 mag (right) of
the NGC 1333 star-forming region surrounding IRAS 2A as observed with the Herschel Space Observatory by the Gould Belt survey (André et al. 2010). The black contour in the maps depicts the 3 mag level. The red curve in the histogram depicts the Gaussian fit of the histogram around the histogram peak.
